The present invention relates generally to optical communications, and more particularly to a pump assembly for Raman amplification with a shared forward propagating pump.
In a long-haul optical transmission system, an optical data signal is transmitted over an optical fiber at distances that typically approach five to ten thousand kilometers. As the optical data signal travels over the optical fiber, the power of the data signal diminishes or attenuates. The attenuation limits the distance the optical data signal can travel to substantially shorter than five thousand kilometers.
To overcome the power loss of the optical data signal, the long-haul optical transmission system typically includes a series of pump modules. The pump modules provide power to be transferred to the optical data signal as it propagates on the optical fiber. This amplification may be implemented using the Raman effect.
Raman amplification uses stimulated Raman scattering to amplify the optical data signal. In stimulated Raman scattering, radiation power from a pump radiation source is transferred to the optical data signal. The Raman gain material for Raman amplification can be the transmission optical fiber itself, known as distributed Raman amplification. Alternatively, it could be a separate fiber chosen specifically for its Raman effect, known as discrete Raman amplification.
The Raman gain coefficient for a silica glass fiber, which is typically used in optical communications, is shown in FIG. 1 as a function of the frequency shift relative to the pump frequency. As shown in FIG. 1, the largest gain occurs at about 13 THz, which corresponds to a 100 nm shift for a 1400 nm pump. Thus, the maximum gain for a single pump wavelength of about 1400 nm will occur at a signal wavelength of about 1500 nm. The gain of the optical data signal from a Raman amplifier is the product of the Raman gain coefficient, the pump intensity, and the length of the interaction medium.
For Raman amplification, the optical data signal is typically amplified by providing a counter-propagating pump in the optical fiber. The counter-propagating pump is generated in a pump module and provides energy for the amplification process. Since the counter-propagating pump attenuates as it travels in the fiber, amplification is greatest near (approximately between 5 and 20 km) and just prior to the pump module. After passing through the pump module, however, the optical data signal is not amplified again until it reaches the length of the optical fiber proximate to the next pump module. As a result, the power of the optical data signal leaving the pump module needs to be relatively high, due to attenuation in the transmission fiber, to enable the optical data signal to reach the length of the optical fiber where it is amplified by the counter-propagating pump of the next pump module.
In addition to amplifying the optical data signal as it propagates on the optical fiber to compensate for attenuation, it is desirable to maximize the optical signal to noise ratio (OSNR) of the optical data signal and to reduce the nonlinearities that affect the transmission of the optical data signal. In particular, as the power of the optical data signal drops, the transmission system OSNR typically drops due to normally constant sources of noise in subsequent optical amplifiers or the photodetection process. It is therefore desirable to have the power be relatively high when the optical data signal leaves the pump module. Increasing the power of the optical data signal, however, increases the nonlinearities that destroy the signal. As a result, the desire to maximize the OSNR and reduce the nonlinearities work against each other.
It is known that a single wavelength pump may be used to provide a forward-propagating pump, also referred to as a co-propagating pump, in conjunction with the counter-propagating pump. The forward-propagating pump reduces the degradation of the OSNR by providing gain to the optical data signal in the length of the optical fiber after the pump module. In addition, the gain provided by the forward-propagating pump reduces the required launch power for the optical data signal, i.e., the power of the optical data signal leaving the pump module. The reduction in launch power reduces the nonlinearities that affect the transmission of the optical data signal.
Briefly, in one aspect of the present invention, a pump assembly for an optical amplifier consistent with the present invention includes a plurality of pump radiation sources, each pump radiation source producing radiation at a respective one of a first number of pump wavelengths, and a coupler, optically coupled to each of the plurality of pump radiation sources, which receives the radiation at each of the first number of pump wavelengths from the plurality of pump radiation sources and outputs the radiation at each of the first number of pump wavelengths to each one of a second number of outputs. The pump assembly also includes a plurality of pump signal combiners, each pump signal combiner optically coupled to a respective one of the second number of outputs of the coupler and receiving the radiation at each of the first number of pump wavelengths output from the coupler, each pump signal combiner placing the radiation at each of the first number of pump wavelengths output from the coupler in co-propagation with a respective one of a plurality of data signals propagating on a respective one of a plurality of optical fibers.
In another aspect of the present invention, the first number and the second number are each at least two, and the first number and the second number are equal or are not equal.
In yet another aspect of the present invention, each of the first number of pump wavelengths have approximately the same wavelength or have different wavelengths, where the difference in wavelength between the shortest pump wavelength and the longest pump wavelength is approximately 10 nm.
In another aspect of the present invention, the first number of pump wavelengths interact with the data signals propagating on the optical fibers to amplify the data signals.